Fuel cells mix a fuel ' usually hydrogen ' with air to produce electricity. On one side of the cell is a source of hydrogen, which is fed through a catalyst-coated membrane sandwiched between two electrodes, the anode and cathode. Hydrogen comes in from the anode, and air is fed through the cathode. The draw of the electrical load catalyzes the hydrogen, forming electrons and protons. The electrons ' which cannot pass through the membrane ' are shuttled away to feed the electrical load; the protons pass through the membrane, combine with the air and produce water vapor.

Fuel cells can be stacked to provide greater voltage if required. When the cartridge's fuel is spent, the user simply replaces it. The cartridge can then be reloaded.

Most portable fuel cells are powered by methane rather than hydrogen. Methane is preferable for portable cells because it can be stored more compactly than hydrogen ' as a liquid ' and converted into hydrogen for the catalyst process.

Hydrogen is usually stored as a gas ' which takes up more space ' so its use in fuel cells is reserved for larger batteries, such as those that one day may power cars.

The advantages of fuel cells are many. They provide uniform voltage at all temperatures, whereas the output of batteries varies in extreme hot or cold weather. Fuel cells are instantly rechargeable by adding a new cartridge. With no moving parts, they're also durable.

Most important, fuel cells have a higher energy density than batteries. According to UltraCell, its UltraCell XX25 cartridge can provide almost 700 watt-hours per kilogram when running at 22 watts-per-hour compared to 150 watt-hours per kilogram provided by Dell M60 laptop batteries. In terms of portability, to carry around 550 watt-hours of juice ' about 25 hours of electricity for a small portable device ' you'd need 7.7 pounds of lithium-ion batteries but only 3.5 pounds of fuel cell cartridges.

' Joab Jackson

'It may not be the generic solution that goes to every infantryman, but some people are willing to take more risk to get more capability.' ' Chris Bolton, chief engineer at the Communications-Electronics Research, Development and Engineering Center's Power Division

Stan Barouh

In today's high-tech Army, soldiers carry around a lot of electronic gadgetry ' a radio, Global Positioning System device, night-vision goggles, maybe even a laptop PC and personal digital assistant. And when the juice runs low, soldiers can't just duck into the nearest coffee shop to recharge. Instead, they must tote around batteries that often weigh more than their rifle ammunition ' about 20 pounds.

To lighten that load, the Army and other branches of the military are looking to fuel cells as a source of power for this multitude of portable devices. Fuel cells generate electricity electrochemically, consuming hydrogen and oxygen and producing water and carbon dioxide as waste products. They offer the advantages of lower weight, longer runtimes and no need for recharging. Like adding gasoline to a car's tank, simply adding more hydrogen ' or rather, a safer, hydrogen-bearing liquid ' keeps the fuel cell running.

Several big players, including Samsung, Sanyo and Toshiba, are developing fuel cells as alternatives to lithium-ion batteries for the commercial market. But other companies are creating products to satisfy the needs of the military.

Soldiers on long missions might need to carry enough batteries to last several days or weeks. The BA-5590, a lithium-ion battery commonly used in the military, weighs about a kilogram and provides about 200 watt-hours of electricity ' enough to power a range of devices for a day or so. And the worst part is that even after those batteries run out, soldiers are still toting around their full weight.

On the other hand, a fuel cell gets lighter as it is used, leaving only the weight of the empty fuel reservoir. Depending on the design, soldiers could refill that reservoir from a larger tank or snap in a replacement fuel cartridge. The Army's goal is a fuel cell that weighs one kilogram or less when empty, said Chris Bolton, chief engineer at the Communications-Electronics Research, Development and Engineering Center's (CERDEC) Power Division.

The technology has progressed rapidly, but fuel cells won't completely replace batteries anytime soon. 'I see niche applications within the military that are very close,' Bolton said. 'There are some people who have very high demands for power over long periods of time. It may not be the generic solution that goes to every infantryman [in the next year], but some people are willing to take more risk to get more capability.'

Fuel cells as powerful as about 25 watts could power a laptop and the portable devices soldiers carry. They likely won't run on pure hydrogen, which is a gas and must be compressed, but rather on a liquid fuel such as methanol or propane. The energy density of liquid fuels can be as high as 1,000 watt-hours per kilogram, Bolton said. Current batteries top out at 250 to 350 watt-hours per kilogram, so fuel cells can provide three to four times the energy for the same weight.

Even devices that don't need to be carried can benefit from longer runtimes. 'In this new warfare, you want fewer people on the ground where they might be vulnerable, so you use sensors to collect data,' said George Relan, vice president of corporate development at MIT Micro. 'If you can double the time between battery changes from two months to four months, you've saved a lot of money on logistics.'

Every branch of the military is interested in fuel cells. The Navy has its eye on them for remote underwater vehicles. The Air Force is looking into fuel cells for applications requiring between 30 and 150 watts of power. Fuel cells at the low end of that range could power portable electronics and on the high end, small, unmanned aircraft used for surveillance. These vehicles have a 6- to 8-foot wingspan, weigh 16 pounds and are launched by hand, like 'throwing a bowling ball into the air,' said Thomas Reitz, chief at the Air Force Research Laboratory's Electrochemical and Thermal Sciences Branch at Wright-Patterson Air Force Base, Ohio. Replacing the batteries with lighter fuel cells would make that job easier.

Ideally, a new fuel cell would meet a broad range of applications, Bolton said. Several companies have focused on direct methanol fuel cells (DMFCs) for low-power devices. Methanol is cheap and easily transported. It is flammable ' like any alcohol ' but it's not as combustible as the compounds in a battery are, Relan said.

The electricity generated by a DMFC comes from an electrochemical reaction between methanol, water and a catalyst-coated membrane. The catalyst strips electrons off the methanol molecules, and the movement of these electrons through the circuit creates electric current, which can be tapped to power devices. From this reaction, carbon dioxide and hydrogen ions are created.

The carbon dioxide is released as a waste product, and the hydrogen ions pass through the membrane to combine with oxygen from the air to form water. Water is needed for the first part of the reaction, so DMFCs are often designed to pump it back across the membrane to mix with methanol.

The chemical reactions that drive a fuel cell are simple, but the engineering needed to make that process efficient is complicated. MTI Micro's research focuses on what Relan calls the engine of the fuel cell ' a chip that packages all the components that allow the electrochemical reactions to take place. MTI Micro's chip pumps water back across the membrane without moving parts, making it easy and inexpensive to mass produce.

PolyFuel has developed a new hydrocarbon material for the fuel cell membrane that reduces what's known as methanol crossover. Conventional membranes tend to allow methanol and water to leak through, which wastes fuel and essentially drowns the fuel cell, said Jim Balcom, president and chief executive officer at PolyFuel. The fuel cell then has to be larger to dissipate heat and function properly ' a drawback if you're trying to make the system portable.

Ultracell has been developing reformed methanol fuel cells (RMFCs) for military applications that run on a 70-30 methanol-water mixture. RMFCs operate at higher temperatures than DMFCs and work in any orientation, said Dan Dalton, who is in charge of government sales at Ultracell. In May, the Army issued a Safety Assessment Report for the company's 25-watt micro fuel cell allowing it to be tested with soldiers. And in June, the fuel cell passed a series of standard military environmental tests designed to evaluate equipment for reliability under harsh conditions.

Other types of fuel cells are being developed too, such as ones that use sodium borohydride as the hydrogen source. The joke, Reitz said, is that 'you can use any fuel, as long as it's hydrogen.' Ideally, the Air Force would like to see fuel cells that run on JP-8 jet fuel because the infrastructure for delivering it to the battlefield is already in place.

'Fuel cells are unlikely to displace batteries in a meaningful way in the next five years,' Reitz said. But he sees promise in hybrid systems that use both fuel cells and batteries to extend runtimes. A radio, for example, could get a steady supply of energy from a fuel cell and short bursts of power from a battery when needed. Or, the fuel cell could serve as the recharger for the battery.

CERDEC works closely with program managers to determine the kinds of power solutions needed, and the soldiers are the final arbiters. 'Ultimately, we'd give it to them in training scenarios, and they would evaluate it,' Bolton said. 'There are a lot of intangible things that may make a unit that is good from the engineering side unsuitable for the soldier.'

Bolton said even though fuel cells are being developed for the military's special needs, having companies develop them for consumer applications will be crucial for long-term success. 'We don't want to be the only buyer of small fuel cells,' he said. 'We're hoping that the industry can get them low-cost enough and reliable enough to establish a commercial market.' Then fuel cells could be bought commercially and used with a few modifications just as batteries are today.